1. Field of the Invention
The present invention relates to a cooling agent, a cooling pack and a cooling box which are used for long-term cooled transportation, cooled storage and cooled display of products (for example, foods, beverages, chemicals, pharmaceuticals, blood, organs, animals and plants) requiring refrigeration or freezing.
2. Description of the Related Art
A sodium chloride aqueous solution or dry ice has been generally used as an ordinary cooling agent.
However, a cooling agent made of a sodium chloride aqueous solution involves the following two problems.
(a) The cooling duration is short.
As will be described later as a result of Comparative Example 2 in FIG. 4, the cooling performance at the initial stage of the use is lost after 12 hours have passed. Accordingly, this cooling agent is not suited for long-term cooling transportation, cooling storage and cooling display.
(b) A supercooling phenomenon occurs.
A supercooling phenomenon means that even after a theoretical temperature of phase transition (solidification point) from liquid to solid is reached when cooling a product, the phenomenon of phase transition (solidification) does not occur as will be described later, for example, as a result of Comparative Example 1 in Table 1. Such a supercooling phenomenon poses various problems including that installation of a high-quality freezer is required, running costs are increased, an operation efficiency is decreased (temperature is decreased by 3% whenever supercooling of 1xc2x0 C. occurs), and extra energy is needed.
Further, a cooling agent made of dry ice involves the following two problems.
(c) The cooling duration is short, and after the cooling duration, the cooling property is abruptly lost.
Dry ice exhibits cryogenic properties, but is sublimated in a short time by releasing carbon dioxide gas. Accordingly, its cooling duration is short. Further, when sublimation is completed, the cooling property is of course abruptly lost.
(d) This cooling agent is harmful to the global environment.
Here, a carbon dioxide gas that causes global warming is released.
It is an object of the invention to provide a cooling agent, a cooling pack and a cooling box which provide a long cooling duration and which are suited for long-term cooled transportation, cooled storage and cooled display (further preferably, with this invention, almost no supercooling phenomenon occurs).
This object is achieved by the following means (1) to (4).
(1) A cooling agent comprising a metal salt aqueous solution, and a nonionic polymer as an agent for extending cooling duration, wherein the nonionic polymer has a group (preferably an amide group) with an affinity due to interaction with a water molecule, and is free of an ionic group that is ionically bound to a metallic ion in the aqueous solution.
Examples of the metal salt can include sodium chloride, ammonium chloride, calcium chloride, magnesium chloride, sodium sulfate, ammonium sulfate, sodium acetate and zinc oxide. An agent can contain at least one metal salt selected from this group.
As the nonionic polymer, poly.N-vinyl acetamide (hereinafter referred to as PNVA), which is a polymer of N-vinyl acetamide, can be exemplified. The basic structure of PNVA is represented by the following formula. 
The amount of PNVA is preferably between 0.5 and 10 parts by weight, more preferably between 1 and 5 parts by weight per 100 parts by weight of a metal salt aqueous solution. When the amount is less than this range, the activity of extending the cooling duration is decreased. When the amount is larger than this range, cost is increased, although the activity of extending the cooling duration is hardly changed at all.
Further, it is preferable to add a supercooling inhibitor to the aqueous solution. Examples of the supercooling inhibitor can include sodium borate, sodium hydrogencarbonate and graphite. An agent can contain at least one compound selected from this group.
(2) A cooling agent comprising a sodium chloride aqueous solution, PNVA as an agent for extending a cooling duration, and sodium borate as a supercooling inhibitor.
The cooling agent is preferably an agent in which a phenomenon of phase transition from liquid to solid occurs in a temperature range of xe2x88x9240xc2x0 to xe2x88x922xc2x0 C. The sodium chloride aqueous solution is preferably an aqueous solution containing 1% to 50% by weight of sodium chloride, more preferably an aqueous solution containing 5% to 30% by weight of sodium chloride.
The amount of PNVA is preferably between 0.5 and 10 parts by weight, more preferably between 1 and 5 parts by weight per 100 parts by weight of the sodium chloride aqueous solution. When the amount is less than this range, the activity of extending the cooling duration is decreased. When the amount is more than this range, cost is increased, although the activity of extending the cooling duration is hardly changed at all.
The amount of sodium borate is preferably between 0.05 and 10 parts by weight, more preferably between 0.1 and 5 parts by weight per 100 parts by weight of the sodium chloride aqueous solution. When the amount is less than this range, the activity of inhibiting supercooling is decreased. When the amount is more than this range, cost is increased, although the activity of inhibiting supercooling is hardly changed at all.
(3) A cooling pack in which the cooling agent recited in (1) or (2) is charged (filled) into a pack made of a resin.
(4) A cooling box in which the cooling pack recited in (3) is placed in a cooling box made of a resin foam.
Further objects of this invention will become evident upon an understanding of the illustrative embodiments described below. Various advantages not specifically referred to herein but within the scope of the instant invention will occur to one skilled in the art upon practice of the presently disclosed invention. The following examples and embodiments are illustrative and not seen to limit the scope of the invention.